The present invention pertains to medication cartridges, and, in particular, to a plunger for a medication cartridge.
Many types of medication are provided in a cartridge including a plunger or piston that is slidable within a barrel of the cartridge. Movement of the plunger within the barrel can force medication from an outlet of the cartridge, such as a needle pierced septum at the cartridge forward end. Such plungers frequently are made of a resilient material with a radial periphery sized and shaped to provide a suitable fluid-tight seal with the interior of the cartridge barrel.
One problem with some resilient plungers is their susceptibility to local compression when acted upon by a drive member that extends into the cartridge barrel to contact and drive forward that plunger to expel the cartridge's medication contents. This compression may adversely impact the suitability of a plunger for certain uses, such as where a larger diameter plunger is needed but the accurate administration of small doses is still required.
To try and address this local compression, variously designed rigid elements have been incorporated into some resilient plungers. The rigid element is usually directly acted upon by the drive member and is sized and shaped to distribute applied load over a larger surface area of the resilient plunger. While useful to distribute loading, using a rigid element is not without its shortcomings.
For instance, a rigid element may force the resilient plunger during use to contact the cartridge barrel in places other than where a fluid tight seal is desired. Such contact results in a frictional resistance to motion of the plunger within the barrel that may, for example, adversely affect the glide force necessary to force movement of the plunger. However, shrinking the diameter of the rigid element, all the other things being equal, can decrease the surface area through which the force is transmitted to the resilient plunger portion, thereby decreasing its force distributing effectiveness. Furthermore, placing a rigid element on the rear end face of a resilient plunger such that it does not extend in a hollow of that plunger, and is less likely to squeeze along the rigid element radial periphery a portion of the resilient plunger against the barrel interior, makes the plunger assembly longer than may be desired.
Thus, it would be desirable to provide a medication cartridge plunger that reduces the chances for an undesired point of sealing contact with the cartridge barrel and to potentially overcome other shortcomings of the prior art.